When water is pumped to the surface of the Earth along with crude petroleum oil, producers often attempt to determine the water content of the oil because water can corrode pipes and damage down-stream processing equipment. Furthermore, water has no value relative to the oil and in-fact can become a disposal or environmental problem wherever it is finally removed.
The accurate determination of the water content and the validation of the amount of water in crude petroleum oil is particularly important during the taxation of crude petroleum oil and the sale of crude petroleum oil, where the owner or seller of the oil does not want to pay taxes on water and the customer does not want to pay the price of oil for water. Such determinations and validations can be conducted on-line and off-line during petroleum processing.
Offline methods involve physically sampling the stream and analyzing it in a laboratory setting. In the petroleum industry, sampling is usually done using a composite sampler, which automatically opens a sample valve attached to a pipeline at a certain time interval to collect an aggregate sample into a sample container. The objective is to collect a sample which is representative of the entire lot of petroleum under consideration. After collection, the composite sample is usually picked up by a person and taken to a laboratory. The composite sample is then “sampled” to prepare aliquots, or sub-divisions of the composite sample, for each of the various characterizations, or analysis methods, being implemented.
However, composite petroleum samplers and the associated analytical methods have problems and disadvantages, such as meeting a desired accuracy for a given determination. For example, results for composite samplers are typically only available at the end of a batch or a test, and there is no recourse if something goes wrong with the sampling system during the sampling process. At the end of the sampling and analysis, only a single number is available to consider. Additionally, the exposure of personnel to hazardous liquids associated with processing the samples is undesirable. Thus, the petroleum industry has continued to seek other methods that provide the required accuracy, speed, and safety.
Accordingly, the use of rapid on-line instruments such as densitometers, capacitance probes, radio frequency probes, and microwave analyzers to measure the water content of petroleum products is becoming more common. In addition to providing increasingly accurate determinations of water content, real time water content results via on-line methods can provide beneficial operational advantages. Knowledge of when water becomes present in petroleum as it is being produced and the magnitude of the quantity of the water may provide an opportunity to remove the water before it reaches and corrodes or damages a transport pipeline, storage vessel, or shipping tanker. Additionally, real time data may show if the water is detected in several short periods of time or if it is present across the entire load of the petroleum. Furthermore, real time analyzers may be used as a comparison to the results provide by composite samplers. Finally, on-line measurements of, for example, physical and electrical properties via instrumentation reduces the need for human involvement in the process of characterizing a multiphase fluid mixture.
Despite the importance and advantages of on-line analyzers, they are typically subjected to long periods of use without recalibration although they may require recalibration due to things such as coating build up, leakage of o-ring seals, voltage changes with time, or component failure. Users of on-line analyzers often do not realize that the system has been compromised and requires recalibration until a manual sample of the multiphase fluid mixture is performed or the values produced by the analyzers are always zero, not changing, or indicate 100% water.